The present invention relates to a process for preparing Ziegler-Natta catalyst systems, which also contain magnesium. The resulting catalyst is useful in a solution polymerization process and for the preparation of polymers of ethylene, especially homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins. In particular, the invention relates to a method of sequentially reacting the components used to make the catalyst using specific holding times, concentrations and temperature conditions to control the activity of the catalyst and the structural properties of the polymers prepared. Catalyst concentration was not considered in the prior art an important variable for controlling the catalyst activity and polymer properties.
Polymers of ethylene, for example, homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins, are used in large volumes for a wide variety of end-uses, for example, in the form of film, fibers, molded or thermoformed articles, pipe, pipe coating and the like. There are two types of processes for the manufacture of polyethylene that involve the polymerization of monomers in an inert liquid medium in the presence of a coordination catalyst viz. those which operate at temperatures below the melting or solubilization temperature of the polymer and those which operate at temperatures above the melting or solubilization temperature of the polymer. The latter are referred to as xe2x80x9csolutionxe2x80x9d processes, an example of which is described in Canadian Patent 660,869 of A. W. Anderson, E. L. Fallwell and J. M. Bruce, which issued Apr. 9, 1963. In a solution process, the process is operated so that both the monomers and polymer are soluble in the reaction medium.
European Patent 0 606 289 B1 granted May 28, 1997 to Zboril describes three procedures for preparing catalyst for solution polymerization process. The difference among the three procedures was the addition sequence of the initial reagents to form the first catalyst component, which is the result of the reaction of magnesium alkyl, first aluminum alkyl, reactive chloride and titanium compound. The second component is formed from the reaction between a second aluminum alkyl and an alcohol. The combination of the first and second components is the final step of the three procedures; however, no claim on catalyst concentration was made in any of the three procedures. In fact it was stated on page 4 at lines 54 through 58 and on page 5 at line 1 that the concentration of the components of the solutions used in the preparation of the catalyst is not critical.
U.S. Pat. No. 5,589,555 issued Dec. 31, 1996 to Zboril et al. describes a process for controlling a solution polymerization process by controlling the composition of the aluminum alkoxy alkyl used in activating the catalyst. The catalyst is prepared reacting a mixture of a dialkyl magnesium compound and a first aluminum compound with a reactive halide and titanium tetrachloride and optionally a vanadium oxytrihalide to make a first component. The resulting mixture is then reacted with a mixture of a second aluminum alkyl compound. The patent does not teach or suggest heating of the reaction product of the first three components to elevated temperatures from 120xc2x0 C. to 180xc2x0 C. before adding a second aluminum component.
The specification teaches at column 2 lines 54 through 64 that, optionally the first component may be prepared by reacting (i) the first aluminum alkyl compound, and (ii) the titanium tetrachloride containing optionally vanadium oxytrihalide at temperature of less than 30xc2x0 C. and heating the resultant admixture to at temperature of 150-300xc2x0 C. for a period of 5 seconds to 60 minutes before adding (iii) second aluminum alkyl. No magnesium compound is used in this option and the catalyst concentration is considered to be not critical.
U.S. Pat. No. 5,492,876 issued Feb. 20, 1996 to Gillis et al. teaches a process to prepare a catalyst comprising mixing in an inert organic solvent, aluminum compound of the formula AIRnX3xe2x88x92n where R is a C1-20 alkyl, cycloalkyl and aryl radical, X is a halogen and n is 1, 2 or 3; with titanium tetrahalide and vanadium oxytrihalide and heating the resulting solution to a temperature from 180xc2x0 C. to 250xc2x0 C. for a time from 15 seconds to 5 minutes and cooling the resulting solution to a temperature of less than 100xc2x0 C. and adding second vanadium oxytrihalide compound to the cooled mixture and then activating the mixture with an additional aluminum compound, preferably an aluminoxane. The present invention does not contemplate the addition of a vanadium oxytrihalide nor does it contemplate the cooling step. Additionally the reference does not teach the presence of a magnesium compound and the use of catalyst concentration as a variable to control catalyst activity and polymer properties.
Typically in the solution polymerization of ethylene catalyst tends to produce lower molecular weight polymer (which is indicated by a higher melt index) and lose activity at high temperatures. Also in producing ethylene copolymers it is desirable to have more and more uniformly distributed co-monomer incorporation which is reflected by a lower density. It is desirable to find a catalyst having a high activity without significantly reducing the molecular weight of the polymer. The present invention seeks to provide such a catalyst.
The present invention provides a process for preparing an olefin polymerization catalyst comprising. sequentially mixing:
(i) a mixture of a compound of the formula (R1)2Mg where in R1 is selected from the group consisting of C1-6 alkyl radicals and a compound of the formula Al1(R2)3 wherein R2 is selected from the group consisting of C1-6 alkyl radicals; and HCl or a reactive organic chloride for example but not limited to R3Cl wherein R3 is selected from the group consisting of C1-6 alkyl radicals and C6-12 aromatic radicals; and maintaining the resulting mixture at a temperature from 25xc2x0 C. to 80xc2x0 C., preferably 40xc2x0 C. to 60xc2x0 C. for a period from 60 to 150 seconds, preferably 90 to 120 seconds, at the concentration of R3Cl adjusted to obtain the desired polymer properties and catalyst performance; depending on the equipment it is between 5 to 100 mmol/L, preferably between 8 and 70 mmol/L; then
(ii) adding a titanium compound of the formula (R4)aTiClbxe2x88x92a wherein R4 is a C1-6 alkoxy radical and a and b are integers from 0 to 4 and the sum of a+b is 4 and holding the mixture for a time from 20 to 90 seconds, preferably from 30 to 50 seconds at a temperature from 25xc2x0 C. to 80xc2x0 C., preferably 40xc2x0 C. to 60xc2x0 C., at the concentration of (R4)aTiClbxe2x88x92a adjusted to obtain the desired polymer properties and catalyst performance; depending on the equipment it is between 0.1 to 10 mmol/L, preferably between 0.1 and 5 mmol/L, most preferably between 0.5 and 5 mmol/L; and
(iii) adding a compound of the formula (R5)2Al2(R6) wherein R5 is a C1-6 alkyl radical and R6 is a C1-6 alkoxy radical and holding the resulting mixture at a temperature from 25xc2x0 C. to 80xc2x0 C., preferably 40xc2x0 C. to 60xc2x0 C. for a period of time from 5 to 30 seconds, preferably from 3 to 15 seconds. The concentration of (R5)2Al2(R6) is not critical and is primarily governed by practical considerations.
The components are added in amounts to provide: an atomic ratio of Mg:Al1 is from 3:1 to 8:1, most preferably about 4.5:1; the atomic ratio of Cl:Mg is from 1.5:1 to 2.5:1, most preferably about 2:1; the atomic ratio of Al1:Ti is from 0.5:1 to 1.5:1, most preferably 0.8:1 to 1.5:1; the atomic ratio of Mg:Ti is from 3:1 to 8:1, preferably from 4:1 to 8:1, most preferably from 4:1 to 6:1; and the atomic ratio of Al2:Ti is from 0.5:1 to 2:1, most preferably from 0.75:1 to 1.5:1.
The present invention also provides a process for the solution polymerization of from 80 to 100% of ethylene with from 0 to 20 weight % of one or more C3-8 alpha-olefins at a temperature from 105xc2x0 C. to 200xc2x0 C. but not limited to, in the presence of a catalyst prepared as described above.
The present invention is directed to a process for the preparation of high molecular weight polymers of alpha-olefins, such polymers being intended for fabrication into articles by extrusion, injection molding, thermoforming, rotational molding and the like. In particular, the polymers of alpha-olefins are homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins i.e. alpha-olefins especially such higher alpha-olefins having 3 to 12 carbon atoms (i.e. C3-12 alpha-olefins), preferably having from 3 to 8 carbon atoms, examples of which are 1-butene, 1-hexene and 1-octene. The preferred higher alpha-olefins have 4 to 10 carbon atoms. Optionally, cyclic endomethylenic dienes may be copolymerized with the ethylene and C3-12 alpha-olefins. Such polymers are known.
In the polymerization process using the catalyst of the present invention, inert hydrocarbon solvent, and optionally hydrogen for control of the polymer molecular weight, are fed to a reactor system together with monomers dissolved in the solvent. The monomer may be ethylene or mixtures of ethylene and at least one C3-12 higher alpha-olefin, preferably ethylene or mixtures of ethylene and at least one C4-10 higher alpha-olefin.
In accordance with the present invention the catalyst is prepared by using the sequence of addition and the heating and holding conditions described above.
In the process of the present invention in step (i) the components are a mixture of a dialkyl magnesium compound and a trialkyl aluminum compound and a halide, preferably an organic halide, most preferably a chloride.
In the process for preparing the catalysts of the present invention alkyl substituents R1, R2, R3 and R5 are preferably selected from the group consisting of C1-4 alkyl radical radicals. Such radicals include methyl, ethyl, propyl and primary, secondary and t-butyl radicals. Preferably each R1 is independently selected from the group consisting of ethyl and butyl, most preferably butyl radicals. Preferably R2 are methyl or ethyl radicals. Preferably R3 is a t-butyl radical. Preferably R5 is an ethyl radical.
In the process for preparing the catalysts of the present invention alkyl substituent R3 is preferably selected from the group consisting of C1-4 alkyl radicals and a benzyl radical. Such radicals include methyl, ethyl, propyl and primary, secondary and t-butyl radicals. Preferably R3 is a t-butyl radical.
In the process of the present invention R4 of the titanium compound used in step (ii) and R6 of the aluminum compound used in step (iii) are preferably selected from the group consisting of C1-4 alkoxy radicals. Such radicals include methoxy, ethoxy, propoxy and butoxy radicals. Most preferably R6 is an ethoxy radical.
In the process for preparing the catalyst of the present invention in the titanium compound used in step (ii) a is preferably 0 and the compound is a titanium tetrahalide, most preferably titanium tetrachloride (i.e. TiCl4).
The catalyst components are mixed at a temperature from 25xc2x0 C. to 80xc2x0 C., preferably 40xc2x0 C. to 60xc2x0 C. The temperature of mixing may be controlled by the use of a jacketed reactor. The jacket may be used to provide or remove heat. The mixing may take place in a continuous stirred tank reactor (CSTR) or in a tubular reactor preferably having an internal mixing system such as an agitator or static mixers. If a tubular reactor is used the mixing may be achieved by operating under turbulent flow conditions. The hold up times may be controlled by the ratio of the rate of flow through the reactor and the length of the reactor. The concentration of each catalyst component in the admixture is controlled by the concentration of the catalyst solution from which the catalyst component is drawn and the flows of solvent and catalyst solution before they are mixed.
In a preferred embodiment of the present invention, in step (i) the hold up time before adding the titanium compound is for a period from 60 to 150 seconds, preferably 90 to 120 seconds, most preferably from 110 to 115 seconds. The mixture of components is held at a temperature from 25xc2x0 C. to 80xc2x0 C., preferably 40xc2x0 C. to 60xc2x0 C. during the hold up time. The concentration of chlorine in the admixture is adjusted to obtain the desired polymer properties and catalyst performance. Depending on the equipment (i.e. in some equipment it may not be practical to approach the lower concentration limit - this is permissible provided the upper limit is not exceeded) it is from 5 to 100 mmol/L, preferably from 8 to 70 mmol/L.
Preferably in step (ii) of the present invention the temperature is from 25xc2x0 C. to 80xc2x0 C., most preferably from 40xc2x0 C. to 60xc2x0 C. and the hold up time before adding the next component is from 20 to 90 seconds, preferably from 30 to 50 seconds; most preferably from 40 to 45 seconds. The concentration of titanium in the resulting mixture is adjusted to obtain the desired properties and catalyst performance. Depending on the equipment (i.e. in some equipment it may not be practical to approach the lower concentration limitxe2x80x94this is permissible provided the upper limit is not exceeded) it is from 0.1 to 10 mmol/L, preferably from 0.1 to 5 mmol/L, most preferably from 0.5 to 5 mmol/L.
In step (iii) of the present invention the temperature is from 25xc2x0 C. to 80xc2x0 C., most preferably from 40xc2x0 C. to 60xc2x0 C., and the hold up time from 3 to 30 seconds, preferably from 3 to 15 seconds, most preferably from 3 to 10 seconds. The concentration of (R5)2Al2(R6) is not critical and is primarily governed by practical considerations.
In a preferred embodiment of the present invention the atomic ratio of Mg:Al1 is from 3:1 to 8:1, most preferably 4:1 to 6:1; the atomic ratio of of Cl:Mg is from 1.5:1 to 2.5:1, most preferably 1.8:1 to 2.2:1; the atomic ratio of A1:Ti is from 0.5:1 to 1.5:1, most preferably 0.8:1 to 1.5:1; the atomic ratio of Mg:Ti is from 3:1 to 8:1, preferably 4:1 to 8:1, most preferably 4:1 to 6:1; and the atomic ratio of Al2:Ti is from 0.5:1 to 2:1, most preferably from 0.75:1 to 1.5:1.
The process for preparing the catalyst may be conducted in a tubular reactor or in a series of continuous stirred tank reactors (CSTR) which are typically jacketed to control the temperature. The output from the tubular reactor or the last CSTR feeds into the polymerization reactor.
The concentrations of the components of the solutions used in the preparation of the catalyst are critical except the concentration of (R5)2Al2(R6), which is not critical and is primarily governed by practical considerations.
As exemplified hereinafter, the sequence of steps, temperature, hold up times and catalysts concentrations in the preparation of the catalyst are important in obtaining a catalyst with high activity. The solvent used in the preparation of the coordination catalyst is an inert hydrocarbon, in particular a hydrocarbon that is inert with respect to the coordination catalyst. Such solvents are known and include for example, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. The solvent used in the preparation of the catalyst is preferably the same as that fed to the reactor for the polymerization process.
The catalyst described herein is used in the process of the present invention without separation of any of the components of the catalyst. In particular, neither liquid nor solid fractions are separated from the catalyst before it is fed to the reactor. In addition, the catalyst components are not slurries. All the components are easy-to-handle, storage stable liquids.
The catalyst described herein may be used according to the process of the present invention, over the wide range of temperatures that may be used in an alpha-olefin polymerization process operated under solution conditions. For example, such polymerization temperatures may be in but not limited to the range of 105xc2x0 C. to 200xc2x0 C. and especially in the range of 130xc2x0 C. to 180xc2x0 C. The polymerization process may be conducted in a reactor system such as in a tubular reactor or multi-reactor system.
The pressures used in the process of the present invention are those known for solution polymerization processes, for example, pressures in the range of about 4-20 MPa.
In the polymerization process of the present invention, the alpha-olefin monomers are polymerized in the reactor in the presence of the catalyst. Pressure and temperature are controlled so that the polymer formed remains in solution.
Optionally, small amounts of hydrogen, for example 1-40 parts per million by weight, based on the total solution fed to the reactor may be added to one or more of the feed streams of the reactor system in order to improve control of the melt index and/or molecular weight distribution and thus aid in the production of a more desirable product, as is disclosed in Canadian Patent 703,704.
The solution passing from the polymerization reactor is normally treated to deactivate any catalyst remaining in the solution. A variety of catalyst deactivators are known, examples of which include but not limited to fatty acids, alkaline earth metal salts of aliphatic carboxylic acids and alcohols. The hydrocarbon solvent used for the deactivator is preferably the same as the solvent used in the polymerization process. If a different solvent is used, it must be compatible with the solvent used in the polymerization mixture and not cause adverse effects on the solvent recovery system associated with the polymerization process.
After deactivation of the catalyst, the solution containing polymer may be passed through a bed of activated alumina or bauxite, which removes part, or all of the deactivated catalyst residues. In a preferred embodiment, the polymerization is conducted without removal of deactivated catalyst residues. The solvent may then be flashed off from the polymer, which subsequently may be extruded into water and cut into pellets or other suitable comminuted shapes. The recovered polymer may then be treated with saturated steam at atmospheric pressure to, for example, reduce the amount of volatile materials and improve polymer color. The treatment may be carried out for about 1 to 16 hours, following which the polymer may be dried and cooled with a stream of air for 1 to 4 hours.
Pigments, antioxidants, UV screeners, hindered amine light stabilizers and other additives may be added to the polymer either before or after the polymer is initially formed into pellets or other comminuted shapes. The antioxidant incorporated into polymer obtained from the process of the present invention may, in embodiments, be a single antioxidant e.g. a hindered phenolic antioxidant, or a mixture of antioxidants e.g. a hindered phenolic antioxidant combined with a secondary antioxidant e.g. a phosphite. Both types of antioxidant are known in the art. For example, the ratio of phenolic antioxidant to secondary antioxidant may be in the range of 0.1:1 to 5:1 with the total amount of antioxidant being in the range of 200 to 3000 ppm.
The polymerization process of the present invention may be used to prepare homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins having densities in the range of, for example, about 0.900-0.970 g/cm3 and especially 0.910-0.930 g/cm3. Such polymers may have a melt index, as measured by the method of ASTM D-1238, condition E, in the range of, for example, about 0.1-200 dg/min, and especially in the range of about 30-45 dg/min. Such a melt index tends to indicate a higher molecular weight of the resulting polymer. The polymers may be manufactured with narrow or broad molecular weight distribution. For example, the polymers may have a stress exponent, a measure of molecular weight distribution, in the range of about 1.1-2.5 and especially in the range of about 1.3-2.0. The process of the invention is believed to be particularly useful in the manufacture of broad molecular distribution polymers.
Stress exponent is determined by measuring the throughput of a melt indexer at two stresses (2160 g and 6480 g loading) using the procedures of the ASTM melt index test method, and the following formula:
Stress exponent=1/0.477 xc3x97log (wt. of polymer extruded with 6480 g wt.)/(wt. of polymer extruded with 2160 g wt.)
Stress exponent values of less than about 1.40 indicate narrow molecular weight distribution while values above about 1.70 indicate broad molecular weight distribution.
The polymers prepared using the catalyst of the present invention tend to have a higher molecular weight than those prepared using the catalysts of the prior art using comparable polymerization conditions. In the alternative, the catalyst of the present invention may be used at higher polymerization temperatures to produce polymers having comparable molecular weights to those produced using prior art catalysts.
The catalysts of the present invention have improved activity over prior Mg/Al/Ti catalysts and particularly those used in solution process at temperatures between but not limited to 105xc2x0 C. and 200xc2x0 C. In a CSTR the catalyst activity is defined as:
xe2x80x83(KP)(HUT)=(Q/(1xe2x88x92Q))(1/Catalyst Concentration)
wherein Q is the fraction of ethylene monomer converted;
HUT is a reciprocal space velocity (hold up time) in the polymerization reactor expressed in minutes and maintained constant throughout the experimental program; and the catalyst concentration is the concentration in the polymerization reactor expressed in mmol of Ti/L.
The polymers produced by the process of the present invention are capable of being fabricated into a wide variety of articles, as is known for homopolymers of ethylene and copolymers of ethylene and higher alpha-olefins.
The process of the present invention is believed to be particularly advantageous in the preparation of broad molecular weight distribution polymers using an adiabatic tubular reactor, or a series of reactors.